Ultrasound transducers used for medical imaging and non-destructive testing are characterized by two main properties, sensitivity and bandwidth, which are directly correlated to the penetration and resolution of the imaging system. It is well known in the art that multilayer piezoelectric structures provide sensitivity enhancement compared to conventional single-layer devices because the multilayer structure reduces the electrical impedance of the piezoelectric ceramic element, e.g., lead zirconate titanate (PZT).
In a multi-layer PZT transducer array, the N layers (N greater than 1) are coupled acoustically in series, so that the xcex/2 resonant thickness (where X is the ultrasound wave-length) is t, the stack thickness. When the polarity of an applied voltage matches the poling direction, the piezoelectric material expands in the thickness direction. Since the electrical polarity is the same as the poling direction for each layer, the layers expand or compress together. For a given applied voltage, the electric field across each layer (thickness t/N) is greater than that for a single-layer transducer (thickness t), resulting in larger acoustic output energy. Electrically, the layers are connected in parallel. Compared to a single-layer device, an N-layer device is essentially the sum of N thinner capacitors in parallel. Since the overall thickness of the structure remains constant for a given frequency of operation, the capacitance of the device increases as a function of N2. Correspondingly, the electrical impedance drops as a function of the inverse of N2.
Since conventional (single-layer) transducer elements tend to have a high electrical impedance compared to that of the cable connecting the element to the console, these conventional transducer elements experience a serious impedance mismatch which limits transfer of electrical power between the element and the console electronics. While this mismatch is undesirable for conventional one-dimensional transducers for which the element impedance is typically several hundred ohms compared to 50 ohms for the cable, it is intolerable for multi-row probes where the element impedance is typically a few thousand ohms. A piezoelectric structure having even a few layers is enough to reduce the mismatch and thereby improve the sensitivity to a tolerable level.
In an ideal piezoelectric element, the electric field is uniform and homogeneous throughout the piezoelectric material. In contrast, for piezoelectric elements having wraparound electrodes, the electric field near the wraparound electrode is distorted. Thus, when a voltage is applied, the resulting electric field produces undesirable stresses in the element. These stresses reduce the desired motion. In particular, the electromechanical efficiency is reduced compared to a parallel plate geometry.
In the past, others have recognized this problem in different contexts and have worked to eliminate it [see, e.g., U.S. Pat. No. 4,217,684] or exploit it to improve contrast resolution in an ultrasound imager [see, e.g., U.S. Pat. No. 4,460,841]. Piezoelectric ceramic has a remarkably high dielectric constant relative to air or most other materials, typically several hundred times greater for the hard PZT""s to several thousand times greater for the soft PZTs. Desilets et al. [xe2x80x9cEffect of Wraparound Electrodes on Ultrasonic Array Performance,xe2x80x9d 1998 IEEE Ultrasonics Symposium] teaches use of a saw kerf (i.e., dicing slot) to change the dielectric constant near the edges of the piezoelectric ceramic layer. This dramatically reduces the fringe electric field with its associated capacitance and also minimizes stresses caused by horizontal components of the electric field. Thus, dicing slots can be used to confine the electric field and create more uniform mechanical motion. However, the kerf cannot be arbitrarily narrow because it must be produced with a saw blade whose thickness is governed by the strength of the blade material. Similarly, the segment of ceramic supporting the wraparound electrodes must be substantial enough not to break. The kerf can be filled with epoxy, or another low dielectric constant material, and still achieve the desired effect. However, the low-dielectric-constant material is introduced after the ceramic structure (having a high dielectric constant) has been otherwise fabricated as a homogeneous body.
All piezoelectric transducers operate by applying a voltage to electrodes on opposite faces of the device. For a single-layer transducer, it is not necessary to have electrodes which wrap around the edge of the piezoelectric ceramic. In certain fabrication strategies it is convenient to have the electrodes available on the side of the elements or to be able to make electrical contact to both the top and bottom of the ceramic from just one of those surfaces.
However, when working with multilayer piezoelectric transducers, there must be a connection to the internal electrodes. This contact is usually provided by a wraparound electrode. Multilayer piezoelectric transducers are most useful for multirow arrays where the element impedance is high, typically over a kilo-ohm. However, a simple dicing cut (as taught by Desilets et al.) would sever the connection to the internal electrodes for these transducers.
Thus there is a need for a technique which would allow one to control the electric field in a multilayer piezoelectric transducer.
An ultrasound transducer array is manufactured by a method that introduces low-delectric-constant material, to confine the electric field, during fabrication of a multilayer piezoelectric ceramic. In the prior art, air or other material with a low dielectric constant was introduced through the thickness of the ceramic after the ceramic had been formed as a homogeneous body. In accordance with the preferred embodiment, edge segments made of low-dielectric-constant material and extending in the thickness direction are formed at opposing ends of the multilayer piezoelectric ceramic structure. These edge segments serve to confine the electric field to keep it substantially uniform and homogeneous throughout the piezoelectric ceramic material.
The low-dielectric-constant material is introduced at the of the multilayer ceramic. Each edge segment made of low-dielectric-constant material is situated and configured to separate a distal edge of an internal electrode from an opposing inter-electrode connection on the side of the piezoelectric ceramic lamination. These low-dielectric-constant regions confine the electric field to the high-dielectric-constant material, where it remains directed vertically. Consequently, when a voltage is applied between the electrodes, the piezoelectrically induced strains are almost entirely vertical. Spurious modes are therefore substantially reduced.
In a preferred embodiment, the low-dielectric-constant edge segments are not strained by an applied voltage. Hence the mode of vibration of the element is modified compared to the parallel plate geometry. This may even result in an improved beam profile since the ultrasound will be apodized, as is well known in the art [see, e.g., U.S. Pat. No. 4,460,841]. A modest broadening of the central lobe of the beam is compensated by significant reductions in sidelobe levels.